Have you ever noticed conical flask necks are not always perfectly round: and slightly oval.Sounds obvious, but I have come from a place that has different sized bungs on delivery tubing to gas syringe because all the conicals had different neck sizes!!!- had to allocate the right ones before sending out a class set..
Sooo much easier if all conicals have same neck size, so easy class sets of delivery tubing/gas syringes etc...
You can discern this if you look carefully (100ml).
Important, when doing eg HCl/Mg rates/concentration and using gas syringes which must have a good seal to work properly.